Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, combustion equipment 15, a high pressure turbine 16, an intermediate pressure turbine 17, a low pressure turbine 18 and an exhaust nozzle 19.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbine 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13, and the fan 12 by suitable interconnecting shafts.
It will be understood that particularly with regard to aircraft engine installations, weight is an important consideration. In such circumstances, if turbine fan blades can be hollow there is a significant reduction in both individual and collective weight of the fan blades and the propulsive fan 12. However, it will also be understood that in view of the significant air flow rates there is a high danger with regard to impacts, particularly upon the fan blades of the propulsive fan 12 at the air intake 11 end of the engine 10. It will be understood that these impacts may be as a result of debris or bird strikes on the propulsive fan 12 and possibly as a result of secondary and cursory impacts through the engine. In such circumstances, the inherent reduced weakness and susceptibility to deformation of hollow fan blades is a significant problem.
It will be understood that the fan blade assembly in the engine must be balanced for appropriate operation and that each individual fan blade must remain sufficiently structurally strong for continued operation until repair is possible. Previously, it has been known to fill the hollow cavity within fan blades with elastomeric materials in order to provide principally vibration damping within the blade but also by implication some reinforcement of that blade. Nevertheless, it will be appreciated that these hollow cavity fillings being of an elastomeric nature will still become distorted with the fan blade when exposed to severe impact loads beyond the elastic deformation response of that fan blade in association with the filling.
As indicated above, the most dangerous impacts to fan blades relate to so-called bird strikes. Such bird strikes alter the run on behaviour of hollow fan blades due to alterations in the propulsive fan configuration which detrimentally affect both structural stiffness and natural frequency within the fan blade. These variations in structural stiffness and frequency combine with leading edge deformations in order to cause blade flutter and possibly further structural degradation due to vibrational work hardening etc. In view of these factors, it is generally accepted good practice to provide a more robust hollow fan blade than strictly necessary in order to resist initial deformation. Clearly, more robust fan blades necessitate greater blade mass for the same material type.